Embodiments of the present disclosure are related to wind turbines, and more particularly to methods and systems to shut down a wind turbine.
Due to various factors including aerodynamic forces, wind turbines may have oscillations. FIG. 1 is a perspective view of an exemplary prior art wind turbine 100 to explain oscillations 112 in the wind turbine 100. The wind turbine 100 includes a tower 102, a rotor 104 having a plurality of blades 106, and a nacelle 108. The tower 102 may be coupled to ground, to an ocean floor, or to a floating foundation using any known securing means, such as bolting, cementing, welding, and so on.
Further, in FIG. 1 reference numeral 110 is generally representative of wind. The wind 110 may have a wind speed (v). Moreover, as the wind 110 blows in the indicated direction, the wind 110 typically imposes an aerodynamic torque (Mz) and an aerodynamic thrust (Fz) on the wind turbine 100. Particularly, the aerodynamic torque (Mz) imposed on the blades 106 may cause the blades 106 to rotate in a direction that is substantially perpendicular to the direction of the wind 110. This motion of the blades 106 is represented in FIG. 1 by an angular rotor speed (ωr) of the rotating blades 106.
The wind 110 imposes the aerodynamic thrust (Fz) perpendicular to the rotor 104, causing a top-portion 103 of the tower 102 to move in a downwind direction 114. As used herein, the term “top-portion of a tower” refers to a portion of a tower of a wind turbine that moves and bends during oscillations in the tower while a base of the tower is fixed. Accordingly, the aerodynamic thrust (Fz) moves the top-portion 103 of the tower 102 towards a downwind direction 114 until a downwind position (shown in FIG. 2) is reached. Furthermore, a restoring force R1z (shown in FIG. 2) moves the top-portion 103 of the tower 102 in an upwind direction 116 until an upwind position (shown in FIG. 2) is reached. The movement of the top-portion 103 of the tower 102 towards the downwind direction 114 and the upwind direction 116 continues resulting in the oscillations 112 in the tower 102. In the presently shown configuration, the oscillations 112, for example, are fore-aft oscillations 112. Hereinafter, the term “oscillations” shall be referred to as “fore-aft oscillations”. Exemplary fore-aft oscillations in the tower 102 are shown with reference to FIG. 2.
Referring now to FIG. 2, a diagrammatic illustration 200 of the tower 102 of the prior art wind turbine 100, referred to in FIG. 1, is shown to explain the fore-aft oscillations 112. Reference numeral 202 shows an original position of the tower 102 when the top-portion 103 of the tower 102 is not deflected or bent. As previously noted with reference to FIG. 1, the wind 110 imposes the aerodynamic force Fz to move the top-portion 103 of the tower 102 in the downwind direction 114 towards a downwind position 204, also referred to herein as “downwind movement”. Accordingly, the aerodynamic force Fz results in deflection of the top-portion 103 of the tower 102 in the downwind direction 114 towards a downwind position 204. After reaching the downwind position 204, a resultant of the restoring force R1z and the aerodynamic force Fz acts on the tower 102 from an opposite direction to move the top-portion 103 of the tower 102 in the upwind direction 116 towards an upwind position 206, also referred to herein as “upwind movement”. The movement of the top-portion 103 of the tower 102 continues between upwind positions and downwind positions. The movement of the top-portion 103 of the tower 102 between the upwind positions and the downwind positions are referred to as the fore-aft oscillations 112. It is noted that while the fore-aft oscillations 112 are explained in association with the wind 110, various other factors may initiate and aggravate the fore-aft oscillations 112.
Wind turbines typically operate in a determined range of wind speeds. Moreover, wind turbines operate optimally in uniform wind conditions. Accordingly, it may not be desirable to operate the wind turbine 100 during gusts or excessive turbulence, excessively high wind speeds or very low wind speeds. In these conditions, the wind turbine 100 is usually shut down. The wind turbine 100 may also be shut down for routine or exceptional maintenance and faults due to actuator/sensor failures in the wind turbine 100 However, the shutdown process of the wind turbine 100 may aggravate the fore-aft oscillations 112 in the wind turbine 100. The aggravated oscillations 112 may induce large structural loads potentially causing wear and damage to the wind turbine 102.
Currently, various techniques are available to shut down a wind turbine. One technique entails pitching blades of the wind turbine from the operating position to a feathered parking position at a uniform rate. This technique, however, can lead to large vibrations in the fore-aft direction. Another technique, commonly referred to as a triple-pitch braking, is often utilized to prevent the large structural loads associated with shutting down the wind turbine. In the triple-pitch approach, the blades are pitched from their operating position to the feathered parking position in three stages. In a first stage, the blades are pitched at a fast rate for a first fixed interval of time, for example 1.5 seconds. Thereafter, during a second stage, the blades are pitched at a slower speed for a second fixed interval of time, for example 1.5 seconds. In addition, in a third stage, the pitching rate is once again increased, until the blades reach the feathered position. Though this technique attempts to obviate the shortcomings of the uniform pitching technique, the triple pitch approach is based on a pre-defined pitching profile and an open-loop controlled approach. Particularly, the pitching rate and the time interval for each stage of the three stages is determined based on worst-case expected behavior over a finite set of wind conditions. Therefore, implementation of the triple pitch approach to shut down the wind turbine may also result in a negative aerodynamic thrust on the wind turbine or reduced thrust when the wind turbine is moving in upwind direction resulting in un-damping of tower and the accompanying drawbacks. Accordingly, the triple triple-pitch approach may sometimes increase aerodynamic loads on the wind turbine, thereby compounding the fore-aft vibration problem.
In addition to these techniques, various closed-loop controller techniques have been employed to shut down the wind turbine. Moreover, these techniques also attempt to obviate the issues associated with shutting down the wind turbine. One such closed-loop technique is commonly referred to as a zero-acceleration approach. In this approach, the blades are pitched towards the feathered position until the aerodynamic thrust on the wind turbine is reduced to zero. Thereafter, the system controls the pitch angle of the blades such that the aerodynamic thrust remains zero until the tower has reached an equilibrium position. Subsequently, the blades are pitched again towards the feathered position. Though this approach may aid in reducing excessive oscillations in the tower, this approach prolongs the shutdown time risking damage to wind turbines.